1. Field of the Invention
The present invention generally relates to the field of semiconductor manufacturing, and more specifically, to a mask used in a semiconductor manufacturing process and methods for making the mask and for the haze monitoring using the mask.
2. Description of the Related Art
During the manufacture of semiconductor integrated circuits, photolithography is a very important process. In the photolithography, a pattern (such as a pattern of metal wires or contact holes) with a predefined design is firstly formed on one or more masks, and the pattern on the mask is then transferred onto a photoresist layer on a wafer using a photolithographic tool (e.g. a step-and-scan photolithographic tool) through an exposure process.
During manufacturing processes, mask contamination has always been a concern. Since the pattern in a mask is repeatedly transferred onto a wafer, mask contamination will cause defects in a large number of semiconductor chips, and severely affects process yield. Among different types of mask contamination, haze is the most common contamination which has the severest effect and is difficult to be avoided. Generally, sulphuric acid is used in mask cleaning, which may cause sulphate ions to be left on the mask. These residual sulphate ions will crystallize and thus produce mask haze during the use of the mask. For example, the residual sulphate ions on the mask will associate with cation such as ammonium ions in the environment, thereby forming a crystal such as ammonium sulphate using a DUV light source. As the crystal grows to a certain size (for example, when its size is equivalent to the pattern size on the mask), the precision of pattern transfer will be affected, which consequently lowers the yield of integrated chips. With the continuous shrinking of the size of semiconductor devices, exposure wavelength in photolithography decreases continuously, and thus the effect by haze becomes increasingly severer. For advanced integrated circuit (IC) processes, since more and more semiconductor layers require the use of a 193 nm exposure wavelength, the probability of occurrences of chip defects caused by mask haze will significantly increase.
Thus, it is necessary to detect contamination (especially, mask haze) on a mask, and then, based on the detection result, to remove the contamination by cleaning once a contamination is found.
A conventional method for detecting mask haze is to employ a dedicated mask detection system, such as STARlight-2™ mask inspection system available from KLA-Tencor Inc. This system can perform offline inspection for mask haze. However, this inspection method is very expensive and time consuming. The inspection of one piece of mask usually takes 2 to 4 hours, which not only decreases the production efficiency of semiconductor devices, but also increases the cost. Moreover, since inspection of the mask is regularly carried out offline, it is hard to find mask haze in time.
Another method for detecting mask haze is to manually inspect whether or not patterns in chip areas of a wafer have suffered from mask haze defects as repeating defects in different shots. However, this method is time consuming, requires additional manpower, and does not provide sufficient accuracy. In addition, this method is only capable of detecting mask haze of large size which has already resulted in chip defects, but fails to detect the mask haze before the mask haze causes chip defects. In practice, when mask haze is detected by means of this method, a large number of defect chips have already been produced, which leads to yield loss.